Day 0
Side view of the orbit · sunlight from the left
What we see from the Northern Hemisphere
New moon
0% lit
Learn more about the science
What causes the phases?
The sun always lights half the moon. As the moon orbits Earth about every 29.5 days, we see different amounts of that lit half. Only our viewing angle changes.
It is not Earth's shadow
Earth's shadow only touches the moon during a lunar eclipse, which is rare. Check the simulation: at a half-lit moon, Earth is not even between the sun and the moon.
Why "first quarter" if it looks half full?
The name comes from the moon's position, not its shape. At first quarter, the moon has traveled one quarter of the way around its orbit.
Check Your Understanding
Stuck on a question? Scroll up and use the simulation to figure it out.
Question 1 of 5